Ion beam implanters are used to treat silicon wafers with an ion beam. Such treatment can be used to produce n or p type extrinsic materials doping or can be used to form passivation layers during fabrication of an integrated circuit.
When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in `n type` extrinsic material wafers. If `p type` extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium are implanted.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion beam implanter includes beam forming and shaping structure extending between an ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes en route to the implantation station. When operating the implanter, this interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
For high current ion implanters (approximately 5 milli-amperes beam current), the wafers at the implantation station are mounted on a surface of a rotating support. As the support rotates, the wafers pass through the ion beam. Ions traveling along the beam path collide with and are implanted in the rotating wafers. A robotic arm withdraws wafers to be treated from a wafer cassette and positions the wafers on the: wafer support surface. After treatment, the robotic arm removes the wafers from the wafer support surface and redeposits the treated wafers in the wafer cassette.
Eaton Corporation, assignee of the present invention, currently sells high current implanters under the product designations NV 10, NV-GSD/200, NV-GSD/160, and NV-GSD/80. Current versions of these model ion implanters include a sector magnet for the purpose of ion species selection. Different species ions are emitted from the ion source. These species have the same charge but have different masses. Current sector magnets produce a dipole magnetic field that disperses particles of different momentum-to-charge ratios to isolate the trajectories of the desired ion species. In addition to the dipole field, it is necessary to produce quadrapole fields within such magnets. These quadrapole fields confine the beam within a practical envelope, and focus the beam into a waist at a location along the beam line that includes a resolving aperture. Only ions having the correct mass remain within the beam downstream from the waist.
A `correct` quadrapole magnetic field strength depends in part, on the tendency of the beam to diverge under the effect of its own space charge density, which depends on parameters such as beam current, energy, mass, as well as beamline parameters such as residual gas composition and pressure. Existing high current implanters must operate with a wide range of parameters, ideally requiring different amounts of focussing for optimal transmission of the ion beam and optimal mass selectivity.
Current sector magnets found in ion implanters are designed with fixed quadrapole focussing strength, selected for best operation with a nominal set of beam parameters. Outside these nominal conditions, particularly at very low energy (less than 10 k volts) and high currents, the performance of these systems is significantly compromised.